To increase thermal efficiency and specific thrust, advanced gas turbine stages are designed to operate at increasingly higher inlet temperatures (Suo, 1985). This increase is made possible by advances in materials such as super alloys and thermal-barrier coatings and by advances in cooling technology such as internal, film, impingement, and other techniques (Suo, 1985; Metzger, 1985; Moffat, 1987). With cooling, inlet temperatures can far exceed allowable material temperatures. Though cooling is an effective way to enable higher inlet temperatures, efficiency considerations demand effective cooling to be accomplished with minimum amount of cooling air since it takes energy to pump the cooling air through the turbine system, which operates at high pressures.
For advanced gas turbines, the first-stage stator and rotor typically require film cooling, which strives to form a blanket of cooler air next to the material surface to insulate the material from the hot gas (Golstein, 1971). Many investigators have studied the effects of design and operating parameters on film cooling. These include film-cooling hole inclinations and length-to-diameter ratios, spacing between holes, geometry of holes including shaped holes, surface curvatures, mainflow turbulence, embedded vortices in the mainflow, and unsteadiness from rotor-stator interactions (see, e.g., reviews by Han et al. (2000), Goldstein (2001), Sundén & Faghri (2001), and Shih & Sultanian (2001); in addition see the comprehensive bibliography provided by Kercher (2003 and 2005)).
Of the previous studies, Kelso & Lim (1996) and Haven et al. (1997) showed the important role played by vortices in the evolution of film-cooling jets. One pair, referred to as the counter-rotating vortices (CRVs), was found to lift the jet off the surface that it is intended to protect and to entrain hot gases underneath it. The other pair, referred to an anti-kidney pair, was shown to have a sense of rotation opposite to that of the CRVs, and so can counteract the undesirable tendencies of the CRVs. Thus, it is of interest to develop strategies to control the formation and strength of these vortices in a way that leads to more effective film cooling.
There are several ways to address this problem. One way that has been proposed by several investigators is to alter the structure of these vortices. These include alterations by using shaped-diffusion holes and slots (e.g., Haven et al. (1997), Hyams et al. (1997), and Thole et al. (1998)), by judicious placement of vortex generators (Haven & Kurosaka (1996)), by constructing tabs at hole exit (Zaman & Foss (2005) and Zaman (1998)), by inserting struts inside film-cooling holes (Shih, et al. (1999)), and by creating a trench about the exit of each film-cooling hole (Bunker (2002)). An alternative way is to prevent the CRVs from entraining hot gases by downstream treatment, and this has not been reported.
Thus, although the problems with cooling surfaces associated with gas turbines have been studied and various improvements proposed, problems remain.